1. Technical Field
The present disclosure relates to a toner, a developer, and an image forming apparatus.
2. Description of the Related Art
In a typical electrophotographic image forming apparatus, an electrically- or magnetically-formed latent image is visualized with toner. Specifically, in electrophotography, an electrostatic latent image is formed on a photoconductor and then developed into a toner image with toner. The toner image is transferred onto a transfer medium such as paper and then fixed thereon. In fixing the toner image on a transfer medium, heat fixing methods such as heat roller fixing method and heat belt fixing method are widely employed because of their high energy efficiency.
In recent years, demand for high-speed-printing and energy-saving image forming apparatus is increasing. In accordance with this demand, toner is required to be fixable at much lower temperatures while providing much higher image quality. One approach for achieving low-temperature fixability of toner involves reducing the softening temperature of the binder resin of the toner. However, such a low softening temperature of the binder resin is likely to cause offset phenomenon in which a part of a toner image is adhered to a surface of a fixing member and then retransferred onto a transfer medium in the fixing process. Reducing the softening temperature of the binder resin also reduces heat-resistant storage stability of the toner. As a result, blocking phenomenon in which toner particles fuse together is caused especially in high-temperature environments. In addition, other problems are likely to occur such that toner fuses to contaminate a developing device or carrier particles, or toner forms its film on a surface of a photoconductor.
As a technique for solving these problems, using crystalline resins for the binder resin of toner is known. Crystalline resins have a property of rapidly softening at the melting point. This property makes it possible to lower fixable temperature of toner.
However, merely blending a crystalline resin with an amorphous resin causes a phase separation. Therefore, when a crystalline resin is used for a binder resin of toner, the toner becomes plastic-deformable due to its softness although having high toughness. The technique of merely using a crystalline resin for the binder resin results in a toner having poor heat-resistant storage stability (blocking resistance). Such a toner aggregates in a toner container or image forming apparatus and cannot be supplied for the development of images, resulting in an abnormal image with a low image density.
On the other hand, a crystalline resin can be finely dispersed in toner in a case in which a binder resin containing the crystalline resin and an amorphous resin is emulsified or melt-kneaded under a load from an external force. However, if heat or an external force is applied again, the dispersibility of the crystalline resin gets worse, causing deterioration in low-temperature fixability and blocking resistance.